Components and methods for use in electro-optic displays

ABSTRACT

An electro-optic display comprises, in order, a backplane comprising a plurality of pixel electrodes; a layer of a solid electro-optic medium; a main adhesive layer; and at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer. The electro-optic layer may be in direct contact with the backplane or separated therefrom by a thin auxiliary layer of adhesive. The main adhesive layer may be colored to provide a color filter array. An inverted front plane laminate useful in forming such a display comprises the same layers except that the backplane is replaced by a release sheet. The display combines good low temperature performance and good resolution at higher temperatures.

REFERENCE TO RELATED APPLICATIONS

This application is a division of copending application Ser. No.11/550,114, filed Oct. 17, 2006 (Publication No. 2007/0109219), which isitself a continuation-in-part of application Ser. No. 10/605,024, filedSep. 2, 2003 (Publication No. 2004/0155857, now U.S. Pat. No. 7,561,324,issued Jul. 14, 2009), which claims benefit of Application Ser. No.60/319,516, filed Sep. 3, 2002.

The aforementioned application Ser. No. 11/550,114 also claims benefitof Application Ser. No. 60/596,743, filed Oct. 18, 2005, and ofcopending Application Ser. No. 60/596,799, filed Oct. 21, 2005.

This application is related to:

-   -   (a) U.S. Pat. No. 6,982,178, issued Jan. 3, 2006 on application        Ser. No. 10/249.957, filed May 22, 2003, which claims benefit of        Application Ser. No. 60/319,300, filed Jun. 10, 2002, and        Application Ser. No. 60/320,186, filed May 12, 2003;    -   (b) Copending application Ser. No. 10/907,065, filed Mar. 18,        2005 (Publication No. 2005/0146774, now U.S. Pat. No.        7,236,292), which is a divisional of the aforementioned        application Ser. No. 10/249,957;    -   (c) U.S. Pat. No. 7,110,164, issued Sep. 19, 2006 on application        Ser. No. 10/904,063, filed Oct. 21, 2004, which is a        continuation-in-part of the aforementioned application Ser. No.        10/249,957 and of the aforementioned application Ser. No.        10/605,024; and    -   (d) U.S. Pat. No. 6,864,875, issued Mar. 8, 2005 on application        Ser. No. 10/145,861, filed May 13, 2002, which is a continuation        of application Ser. No. 09/436,303, filed Nov. 8, 1999 (now        abandoned), which is itself a divisional of application Ser. No.        09/289,507, filed Apr. 9, 1999 (now U.S. Pat. No. 7,075,502,        issued Jul. 11, 2006).

The entire contents of these patents and applications, and of all otherU.S. patents and published and copending applications mentioned below,are herein incorporated by reference.

BACKGROUND OF INVENTION

The present invention relates to components and methods for use inelectro-optic displays. Some of the displays produced in accordance withthis invention are color displays. This invention relates primarily tosuch components and methods for forming electro-optic displayscontaining an electro-optic medium which is a solid (such displays mayhereinafter for convenience be referred to as “solid electro-opticdisplays”), in the sense that the electro-optic medium has solidexternal surfaces, although the medium may, and often does, haveinternal liquid- or gas-filled spaces, and to methods for assemblingdisplays using such an electro-optic medium. Thus, the term “solidelectro-optic displays” includes encapsulated electrophoretic displays,encapsulated liquid crystal displays, and other types of displaysdiscussed below.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin published U.S. Patent Application No. 2002/0180687 that someparticle-based electrophoretic displays capable of gray scale are stablenot only in their extreme black and white states but also in theirintermediate gray states, and the same is true of some other types ofelectro-optic displays. This type of display is properly called“multi-stable” rather than bistable, although for convenience the term“bistable” may be used herein to cover both bistable and multi-stabledisplays.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedto applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870.657; and 6,950,220. This type of medium isalso typically bistable.

Another type of electro-optic display, which has been the subject ofintense research and development for a number of years, is theparticle-based electrophoretic display, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field. Electrophoretic displays can have attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays.Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. PatentPublication No. 2005/0001810; European Patent Applications 1,462,847;1,482,354; 1,484,635; 1,500,971; 1,501,194; 1,536,271; 1,542,067;1,577,702; 1,577,703; and 1,598,694; and International Applications WO2004/090626; WO 2004/079442; and WO 2004/001498. Such gas-basedelectrophoretic media appear to be susceptible to the same types ofproblems due to particle settling as liquid-based electrophoretic media,when the media are used in an orientation which permits such settling,for example in a sign where the medium is disposed in a vertical plane.Indeed, particle settling appears to be a more serious problem ingas-based electrophoretic media than in liquid-based ones, since thelower viscosity of gaseous suspending fluids as compared with liquidones allows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspending medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545;6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333;6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050;6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068;6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279;6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851;6,922,276; 6,950,200; 6,958,848; 6,967,640; 6,982,178; 6,987,603;6,995,550; 7,002,728; 7,012,600; 7,012,735; 7,023,430; 7,030,412;7,030,854; 7,034,783; 7,038,655; 7,061,663; 7,071,913; 7,075,502;7,075,703; 7,079,305; 7,106,296; 7,109,968; 7,110,163; 7,110,164;7,116,318; 7,116,466; 7,119,759; and 7,119,772; and U.S. PatentApplications Publication Nos. 2002/0060321; 2002/0090980; 2002/0180687;2003/0011560; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0014265;2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; 2004/0119681;2004/0136048; 2004/0155857; 2004/0180476; 2004/0190114; 2004/0196215;2004/0226820; 2004/0239614; 2004/0257635; 2004/0263947; 2005/0000813;2005/0007336; 2005/0012980; 2005/0017944; 2005/0018273; 2005/0024353;2005/0062714; 2005/0067656; 2005/0078099; 2005/0099672; 2005/0122284;2005/0122306; 2005/0122563; 2005/0122565; 2005/0134554; 2005/0146774;2005/0151709; 2005/0152018; 2005/0152022; 2005/0156340; 2005/0168799;2005/0179642; 2005/0190137; 2005/0212747; 2005/0213191; 2005/0219184;2005/0253777; 2005/0270261; 2005/0280626; 2006/0007527; 2006/0024437;2006/0038772; 2006/0139308; 2006/0139310; 2006/0139311; 2006/0176267;2006/0181492; 2006/0181504; 2006/0194619; 2006/0197736; 2006/0197737;2006/0197738; 2006/0198014; 2006/0202949; and 2006/0209388; andInternational Applications Publication Nos. WO 00/38000; WO 00/36560; WO00/67110; and WO 01/07961; and European Patents Nos. 1,099,207 B1; and1,145,072 B1.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, theaforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat.Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856.Dielectrophoretic displays, which are similar to electrophoreticdisplays but rely upon variations in electric field strength, canoperate in a similar mode; see U.S. Pat. No. 4,418,346.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes; andother similar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, International Application Publication No. WO 02/01281, andpublished US Application No. 2002/0075556, both assigned to SipixImaging, Inc.

Other types of electro-optic media may also be used in the displays ofthe present invention.

An electro-optic display normally comprises a layer of electro-opticmaterial and at least two other layers disposed on opposed sides of theelectro-optic material, one of these two layers being an electrodelayer. In most such displays both the layers are electrode layers, andone or both of the electrode layers are patterned to define the pixelsof the display. For example, one electrode layer may be patterned intoelongate row electrodes and the other into elongate column electrodesrunning at right angles to the row electrodes, the pixels being definedby the intersections of the row and column electrodes. Alternatively,and more commonly, one electrode layer has the form of a singlecontinuous electrode and the other electrode layer is patterned into amatrix of pixel electrodes, each of which defines one pixel of thedisplay. In another type of electro-optic display, which is intended foruse with a stylus, print head or similar movable electrode separate fromthe display, only one of the layers adjacent the electro-optic layercomprises an electrode, the layer on the opposed side of theelectro-optic layer typically being a protective layer intended toprevent the movable electrode damaging the electro-optic layer.

The manufacture of a three-layer electro-optic display normally involvesat least one lamination operation. For example, in several of theaforementioned MIT and E Ink patents and applications, there isdescribed a process for manufacturing an encapsulated electrophoreticdisplay in which an encapsulated electrophoretic medium comprisingcapsules in a binder is coated on to a flexible substrate comprisingindium-tin-oxide (ITO) or a similar conductive coating (which acts as anone electrode of the final display) on a plastic film, thecapsules/binder coating being dried to form a coherent layer of theelectrophoretic medium firmly adhered to the substrate. Separately, abackplane, containing an array of pixel electrodes and an appropriatearrangement of conductors to connect the pixel electrodes to drivecircuitry, is prepared. To form the final display, the substrate havingthe capsule/binder layer thereon is laminated to the backplane using alamination adhesive. (A very similar process can be used to prepare anelectrophoretic display usable with a stylus or similar movableelectrode by replacing the backplane with a simple protective layer,such as a plastic film, over which the stylus or other movable electrodecan slide.) In one preferred form of such a process, the backplane isitself flexible and is prepared by printing the pixel electrodes andconductors on a plastic film or other flexible substrate. The obviouslamination technique for mass production of displays by this process isroll lamination using a lamination adhesive. Similar manufacturingtechniques can be used with other types of electro-optic displays. Forexample, a microcell electrophoretic medium or a rotating bichromalmember medium may be laminated to a backplane in substantially the samemanner as an encapsulated electrophoretic medium.

As discussed in the aforementioned U.S. Pat. No. 6,982,178, many of thecomponents used in solid electro-optic displays, and the methods used tomanufacture such displays, are derived from technology used in liquidcrystal displays (LCD's), which are of course also electro-opticdisplays, though using a liquid rather than a solid medium. For example,solid electro-optic displays may make use of an active matrix backplanecomprising an array of transistors or diodes and a corresponding arrayof pixel electrodes, and a “continuous” front electrode (in the sense ofan electrode which extends over multiple pixels and typically the wholedisplay) on a transparent substrate, these components being essentiallythe same as in LCD's. However, the methods used for assembling LCD'scannot be used with solid electro-optic displays. LCD's are normallyassembled by forming the backplane and front electrode on separate glasssubstrates, then adhesively securing these components together leaving asmall aperture between them, placing the resultant assembly undervacuum, and immersing the assembly in a bath of the liquid crystal, sothat the liquid crystal flows through the aperture between the backplaneand the front electrode. Finally, with the liquid crystal in place, theaperture is sealed to provide the final display.

This LCD assembly process cannot readily be transferred to solidelectro-optic displays. Because the electro-optic material is solid, itmust be present between the backplane and the front electrode beforethese two integers are secured to each other. Furthermore, in contrastto a liquid crystal material, which is simply placed between the frontelectrode and the backplane without being attached to either, a solidelectro-optic medium normally needs to be secured to both; in most casesthe solid electro-optic medium is formed on the front electrode, sincethis is generally easier than forming the medium on thecircuitry-containing backplane, and the front electrode/electro-opticmedium combination is then laminated to the backplane, typically bycovering the entire surface of the electro-optic medium with an adhesiveand laminating under heat, pressure and possibly vacuum.

As discussed in the aforementioned U.S. Pat. No. 6,312,304, themanufacture of solid electro-optic displays also presents problems inthat the optical components (the electro-optic medium) and theelectronic components (in the backplane) have differing performancecriteria. For example, it is desirable for the optical components tooptimize reflectivity, contrast ratio and response time, while it isdesirable for the electronic components to optimize conductivity,voltage-current relationship, and capacitance, or to possess memory,logic, or other higher-order electronic device capabilities. Therefore,a process for manufacturing an optical component may not be ideal formanufacturing an electronic component, and vice versa. For example, aprocess for manufacturing an electronic component can involve processingunder high temperatures. The processing temperature can be in the rangefrom about 300° C. to about 600° C. Subjecting many optical componentsto such high temperatures, however, can be harmful to the opticalcomponents by degrading the electro-optic medium chemically or bycausing mechanical damage.

This patent describes a method of manufacturing an electro-optic displaycomprising providing a modulating layer including a first substrate andan electro-optic material provided adjacent the first substrate, themodulating layer being capable of changing a visual state uponapplication of an electric field; providing a pixel layer comprising asecond substrate, a plurality of pixel electrodes provided on a frontsurface of the second substrate and a plurality of contact pads providedon a rear surface of the second substrate, each pixel electrode beingconnected to a contact pad through a via extending through the secondsubstrate; providing a circuit layer including a third substrate and atleast one circuit element; and laminating the modulating layer, thepixel layer, and the circuit layer to form the electro-optic display.

Electro-optic displays are often costly; for example, the cost of thecolor LCD found in a portable computer is typically a substantialfraction of the entire cost of the computer. As the use of electro-opticdisplays spreads to devices, such as cellular telephones and personaldigital assistants (PDA's), much less costly than portable computers,there is great pressure to reduce the costs of such displays. Theability to form layers of some solid electro-optic media by printingtechniques on flexible substrates, as discussed above, opens up thepossibility of reducing the cost of electro-optic components of displaysby using mass production techniques such as roll-to-roll coating usingcommercial equipment used for the production of coated papers, polymericfilms and similar media. However, such equipment is costly and the areasof electro-optic media presently sold may be insufficient to justifydedicated equipment, so that it may typically be necessary to transportthe coated medium from a commercial coating plant to the plant used forfinal assembly of electro-optic displays without damage to therelatively fragile layer of electro-optic medium.

Also, most prior art methods for final lamination of electro-opticdisplays are essentially batch methods in which the electro-opticmedium, the lamination adhesive and the backplane are only broughttogether immediately prior to final assembly, and it is desirable toprovide methods better adapted for mass production.

The aforementioned U.S. Pat. No. 6,982,178 describes a method ofassembling a solid electro-optic display (including a particle-basedelectrophoretic display) which is well adapted for mass production.Essentially, this patent describes a so-called “front plane laminate”(“FPL”) which comprises, in order, a light-transmissiveelectrically-conductive layer; a layer of a solid electro-optic mediumin electrical contact with the electrically-conductive layer; anadhesive layer; and a release sheet. Typically, the light-transmissiveelectrically-conductive layer will be carried on a light-transmissivesubstrate, which is preferably flexible, in the sense that the substratecan be manually wrapped around a drum (say) 10 inches (254 mm) indiameter without permanent deformation. The term “light-transmissive” isused in this patent and herein to mean that the layer thus designatedtransmits sufficient light to enable an observer, looking through thatlayer, to observe the change in display states of the electro-opticmedium, which will be normally be viewed through theelectrically-conductive layer and adjacent substrate (if present). Thesubstrate will be typically be a polymeric film, and will normally havea thickness in the range of about 1 to about 25 mil (25 to 634 gm),preferably about 2 to about 10 mil (51 to 254 μm). Theelectrically-conductive layer is conveniently a thin metal or metaloxide layer of, for example, aluminum or ITO, or may be a conductivepolymer. Poly(ethylene terephthalate) (PET) films coated with aluminumor ITO are available commercially, for example as “aluminized Mylar”(“Mylar” is a Registered Trade Mark) from E.I. du Pont de Nemours &Company, Wilmington, Del., and such commercial materials may be usedwith good results in the front plane laminate.

Assembly of an electro-optic display using such a front plane laminatemay be effected by removing the release sheet from the front planelaminate and contacting the adhesive layer with the backplane underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electro-opticmedium and electrically-conductive layer to the backplane. This processis well-adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

The aforementioned U.S. Pat. No. 6,982,178 also describes a method fortesting the electro-optic medium in a front plane laminate prior toincorporation of the front plane laminate into a display. In thistesting method, the release sheet is provided with an electricallyconductive layer, and a voltage sufficient to change the optical stateof the electro-optic medium is applied between this electricallyconductive layer and the electrically conductive layer on the opposedside of the electro-optic medium. Observation of the electro-opticmedium will then reveal any faults in the medium, thus avoidinglaminating faulty electro-optic medium into a display, with theresultant cost of scrapping the entire display, not merely the faultyfront plane laminate.

The aforementioned U.S. Pat. No. 6,982,178 also describes a secondmethod for testing the electro-optic medium in a front plane laminate byplacing an electrostatic charge on the release sheet, thus forming animage on the electro-optic medium. This image is then observed in thesame way as before to detect any faults in the electro-optic medium.

The aforementioned 2004/0155857 describes a so-called “double releasefilm” which is essentially a simplified version of the front planelaminate of the aforementioned U.S. Pat. No. 6,982,178. One form of thedouble release sheet comprises a layer of a solid electro-optic mediumsandwiched between two adhesive layers, one or both of the adhesivelayers being covered by a release sheet. Another form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two release sheets. Both forms of the double releasefilm are intended for use in a process generally similar to the processfor assembling an electro-optic display from a front plane laminatealready described, but involving two separate laminations; typically, ina first lamination the double release sheet is laminated to a frontelectrode to form a front sub-assembly, and then in a second laminationthe front sub-assembly is laminated to a backplane to form the finaldisplay.

Electro-optic displays manufactured using the aforementioned front planelaminates or double release films have a layer of lamination adhesivebetween the electro-optic layer itself and the backplane, and thepresence of this lamination adhesive layer affects the electro-opticcharacteristics of the displays. In particular, the electricalconductivity of the lamination adhesive layer affects both the lowtemperature performance and the resolution of the display. The lowtemperature performance of the display can (it has been foundempirically) be improved by increasing the conductivity of thelamination adhesive layer, for example by doping the layer withtetrabutylammonium hexafluorophosphate or other materials as describedin the aforementioned U.S. Pat. No. 7,012,735 and Publication No.2005/0122565. However, increasing the conductivity of the laminationadhesive layer in this manner tends to increase pixel blooming (aphenomenon whereby the area of the electro-optic layer which changesoptical state in response to change of voltage at a pixel electrode islarger than the pixel electrode itself), and this blooming tends toreduce the resolution of the display. Hence, this type of displayapparently intrinsically requires a compromise between low temperatureperformance and display resolution, and in practice it is usually thelow temperature performance which is sacrificed.

This variation of low temperature performance and display resolutionwith lamination adhesive conductivity may be understood in terms of astacked resistor model, whereby the electro-optic layer and thelamination adhesive layer are modeled as two resistors connected inseries between the display electrodes. As the conductivity of thelamination adhesive is increased, more of the voltage applied betweenthe electrodes is dropped across the electro-optic layer. When theconductivity of the lamination adhesive layer is more than about 10times that of the electro-optic layer, essentially the full value of theapplied voltage is used to switch the electro-optic layer, so furtherincreases in lamination adhesive conductivity do not improveelectro-optic performance. However, the lamination adhesive conductivitycannot be made too high, since then the lamination adhesive cannotmaintain lateral differences in potential, with the result thatresolution is lost, with at least part of the spatial information in thebackplane destroyed by the lamination adhesive shorting between adjacentelectrodes.

At least when the electro-optic medium is an encapsulatedelectrophoretic medium, for all known useful lamination adhesives, thetemperature dependence of the conductivity of the lamination adhesive isgreater than that of the electro-optic layer. The conductivity of bothlayers decreases with temperature, but that of the lamination adhesivedecreases more rapidly. If the lamination adhesive were formulated sothat it was only just capable of providing good electro-opticperformance at room temperature, as the temperature was reduced thelamination adhesive would rapidly become less conductive than theelectro-optic layer. Under these conditions, the applied voltage isdivided such that very little potential drop occurs across theelectro-optic layer; instead, most of the potential drop occurs acrossthe lamination adhesive layer, and hence does not contribute toswitching of the electro-optic layer.

Hence, there is a need for an electro-optic display with improved lowtemperature performance without compromising the resolution of thedisplay, and the present invention seeks to provide such anelectro-optic display and components and methods for use in themanufacture thereof

Preferred forms of the present invention can also assist in theproduction of color electro-optic displays. Most types of electro-opticmedia have only a limited number of optical states, for example a dark(black) state, a light (white) state and, in some cases, one or moreintermediate gray states. Accordingly, to construct a full color displayusing such media, it is common practice to place an electro-optic mediumadjacent a color filter array having, for example, multiple red, greenand blue areas, and to provide a driving arrangement for theelectro-optic medium which permits independent control of the mediumadjacent each red, green or blue area. Certain applications of colorfilter arrays with electrophoretic displays are described in theaforementioned U.S. Pat. No. 6,864,875. The aforementioned 2003/0011560describes ways for modifying the optical properties of electrophoreticdisplays by incorporating an optical biasing element in any one ofseveral components of the display.

The present invention seeks to provide improvements in colorelectro-optic displays and in processes for the production of suchdisplays.

SUMMARY OF THE INVENTION

Accordingly, in one aspect this invention provides an electro-opticdisplay comprising, in order:

-   -   a backplane comprising a plurality of pixel electrodes;    -   a layer of a solid electro-optic medium;    -   an adhesive layer; and    -   at least one of a light-transmissive protective layer and a        light-transmissive electrically-conductive layer.

In one form of this electro-optic display of the present invention, theelectro-optic layer is in direct contact with the backplane. In anotherform of this electro-optic display, an auxiliary adhesive layer isinterposed between the backplane and the electro-optic layer. Forreasons explained below, this auxiliary adhesive layer typically has athickness not greater than about 10 μm and/or not greater than about onehalf of the thickness of the (main) adhesive layer on the opposed sideof the electro-optic layer.

The adhesive layer in the electro-optic display of the present inventioncan be used to provide a convenient and flexible color filter array. Forthis purpose, the adhesive layer may comprise at least two sectionshaving differing colors; to provide a full color display, the adhesiveshould have sections having at least three different colors, for exampleyellow, cyan and magenta, or red, green and blue, or red, green, blueand clear sections. As discussed in more detail below, the adhesivelayer is desirably colored using at least one pigment; this pigment mayhave an average particle size in the range of about 5 to about 50 nm.The adhesive layer may comprise other additives, such as an ultra-violetabsorber and/or a light-scattering or light-diffusing material.

The electro-optic display of the present invention may comprise both alight-transmissive protective layer and a light-transmissiveelectrically-conductive layer, the electrically-conductive layer beingdisposed between the protective layer and the adhesive layer. Theelectro-optic display may use of any of the types of solid electro-opticmedia described above. Thus, the present display may comprise anelectrochromic or rotating bichromal member medium, or anelectrophoretic medium having a plurality of electrically chargedparticles disposed in a fluid and capable of moving through the fluid onapplication of an electric field to the electro-optic medium. If anelectrophoretic medium is used, the electrically charged particles andthe fluid may be encapsulated within a plurality of capsules or cells,or may be present as a plurality of discrete droplets held within apolymeric continuous phase. The fluid may be liquid or gaseous.

In another aspect, this invention provides an article of manufacturecomprising, in order:

-   -   a release sheet;    -   a layer of a solid electro-optic medium;    -   an adhesive layer; and    -   at least one of a light-transmissive protective layer and a        light-transmissive electrically-conductive layer.

For reasons discussed below, this article of manufacture may hereinafterbe called an “inverted front plane laminate” or “inverted FPL” of thepresent invention.

This inverted front plane laminate of the present invention may includeany of the optional features of the electro-optic display of the presentinvention, as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic section through a first electro-optic display ofthe present invention.

FIG. 2 is a schematic section through a second electro-optic display ofthe present invention in which the adhesive layer is modified to providea color filter array.

FIG. 3 is a schematic section through third electro-optic display of thepresent invention which is similar to the second display shown in FIG. 2but has an auxiliary adhesive layer between the electro-optic layer andthe backplane.

FIG. 4 is a bar graph showing the dynamic ranges measured in Example 2below.

FIG. 5 is a photograph of the displays used in the resolution testscarried out in Example 3 below.

DETAILED DESCRIPTION

As already indicated, the present invention can alleviate, or eveneliminate, the compromise between resolution and low temperatureperformance hitherto experienced in electro-optic displays by invertingthe order of the electro-optic layer and the lamination adhesive layerin the stack forming the final display, so that the high resolution partof the display (for example the backplane, especially a thin filmtransistor (TFT) backplane) is in direct contact with the electro-opticlayer, or is separated therefrom only by an auxiliary adhesive layerhaving only a small thickness. With a display of this structure, thedisplay resolution is independent of the lamination adhesiveconductivity, so that a lamination adhesive sufficiently conductive tohave good low temperature performance can be used without compromisingdisplay resolution.

The adhesive layer which is present between the electro-optic layer andthe front electrode or front protective layer in the display of thepresent invention can provide a convenient color filter array.

FIG. 1 of the accompanying drawings is a highly schematic cross-sectionthrough a first display of the present invention. This display(generally designated 100) comprises a backplane 102 which comprises aplurality of pixel electrodes and may be of any conventional type, forexample a TFT active matrix backplane, or a direct drive backplane inwhich each pixel electrode is provided with a separate voltage supplyline so that a controller (not shown) can control the voltage of eachpixel electrode independently. An electro-optic layer 104, which may beof any of the types discussed above, is in direct contact with thebackplane 102; alternatively a thin (typically less than 10 μm, or lessthan half the thickness of the main layer of lamination adhesivedescribed below) auxiliary layer of lamination adhesive (not shown) maybe provided between the backplane 102 and the electro-optic layer 104.On the opposed side of the electro-optic layer 104 from the backplane102 is disposed a main lamination adhesive layer 106, which can bechosen to give good low temperature performance, and may, for example,be a highly doped polyurethane adhesive. The last two layers of thedisplay 100 are a front light-transmissive electrically-conductiveelectrode layer 108 and a light-transmissive protective layer 110; asdiscussed In the aforementioned U.S. Pat. Nos. 6,982,178 and 7,110,164,and Publication No. 2004/0155857, the layers 108 and 110 areconveniently supplied using commercially available polymer films coatedwith very thin conductive layers, for example poly(ethyleneterephthalate) (PET) films coated with indium tin oxide (ITO) oraluminum.

As indicated above, only one of the layers 108 and 110 need be presentin the display or inverted front plane laminate of the presentinvention. At least in theory, if the electrode layer 108 issufficiently mechanically robust to survive normal handling, theprotective layer 110 can be omitted; in practice, however,light-transmissive electrodes are normally so thin that some form ofprotective layer is required. To provide the maximum voltage drop acrossthe electro-optic layer 104 and hence the fastest switching speed, theprotective layer should of course be disposed on the opposed side of theelectrode layer from the electro-optic layer. In certain types ofdisplays, for example those intended for use with a stylus or anexternal print head, the electrode layer 108 may be omitted.

FIG. 2 of the accompanying drawings illustrates a second display(generally designated 200) of the present invention, this display beinggenerally similar to that of FIG. 1 but being intended to display fullcolor images. The display 200 comprises a backplane 202, which again canbe of any conventional type. The backplane 202 is illustrated ascomprising three pixel electrodes 203R, 203G and 203B. An electro-opticlayer 104 identical to that shown in FIG. 1, is in direct contact withthe backplane 202. On the opposed side of the electro-optic layer 104from the backplane 202 is disposed a main lamination adhesive layer,which is colored to form red, green and blue strips 206R, 206G and 206Brespectively, these strips being aligned with the corresponding pixelelectrodes 203R, 203G and 203B respectively; as is well known to thoseskilled in display technology, such alignment of the various coloredstrips with the pixel electrodes is necessary to ensure that colors canbe written on the displays independently of one another, as required foraccurate color reproduction. Thus the lamination adhesive layer alsoserves as a color filter array. The last two layers of the display are afront light-transmissive electrically-conductive electrode layer 108 anda light-transmissive protective layer 110, identical to those shown inFIG. 1.

FIG. 3 of the accompanying drawings shows a schematic section through athird display (generally designated 300) of the present invention whichis generally similar to the display 200 shown in FIG. 2 but in which athin auxiliary adhesive layer 312 is disposed between the backplane 202and the electro-optic layer 104. This auxiliary adhesive layer 312 isnot colored; since most electro-optic media are opaque, the auxiliaryadhesive layer 312 is not visible to an observer viewing the display 300through the protective layer 110 and hence the auxiliary adhesive layer312 normally cannot function as a color filter array. However, if forexample the electro-optic layer 104 were intended to operate in shuttermode and the backplane 202 made light-transmissive so that the display300 could be viewed in transmission, the auxiliary adhesive layer couldbe used as a color filter array.

Displays having the structures shown in FIGS. 1 to 3, and similarstructures, can be manufactured in a number of ways. One approach tomanufacturing such displays is to coat the electro-optic layer directlyon to the backplane, where the electro-optic layer is of a type (forexample, an encapsulated electrophoretic layer) which permits suchcoating. In most cases, this approach is not preferred since incommercial practice it requires individual processing of a large numberof discrete, high value components, i.e., the individual backplanes, andthis is awkward to carry out, since coating is more easily and moreeconomically effected on a roll-to-roll basis, or at least on large flatsubstrates, where bar coating or hopper coating can be used, than on alarge number of small separate backplanes.

In most cases, it is more convenient to coat the electro-optic layer onto a release sheet (i.e., a disposable sheet covered with a releaselayer). The resultant electro-optic medium/release sheet subassembly maythen be laminated to a layer of lamination adhesive, coated either on toa second release sheet or on to a conductive, transparent electrode or atransparent protective layer, for example the aforementioned PET/ITOfilm. If the layer of lamination adhesive is an electrode or protectivelayer, the resulting structure is an inverted front plane laminate ofthe invention, so-called because it is essentially identical to thefront plane laminate described in the aforementioned U.S. Pat. No.6,982,178, but with the order of the electro-optic and laminationadhesive layers inverted.

Such an inverted FPL is then used to form a final display by removingthe release sheet adjacent the electro-optic layer, and laminating theremaining layers to a backplane. If the backplane is sufficientlysmooth, and close attention is paid to the lamination conditions used,good, void-free lamination should be achievable, and the resultingdisplay will show both good low temperature performance and highresolution. If void formation (i.e., areas where the electro-opticmedium fails to adhere to the backplane) is found to be a problem, therelease sheet adjacent the electro-optic layer can be removed from theinverted FPL and the remaining layers laminated to a thin layer oflamination adhesive previously coated on to a separate release sheet,thus forming a modified inverted FPL containing a auxiliary layer oflamination adhesive. After removal of the release sheet covering theauxiliary adhesive layer, the modified inverted FPL can be laminated toa backplane in the same manner as previously described, with improvedadhesion to the backplane. Because the surface of the electro-opticlayer exposed by removal of the release sheet will be very smooth (sincethe electro-optic layer was coated on a smooth support), a very thinauxiliary layer (in some cases as little as 1 μm or less) of laminationadhesive will suffice in most cases. This small thickness of adhesivewill not be sufficient to affect either the electro-optic performance orthe resolution of the display. In fact, any thickness of the auxiliarylamination adhesive layer that is less than that of the main laminationadhesive layer should give some improvement in performance. Theconductivity of the auxiliary lamination adhesive layer can be varied ifdesired. The thicker this auxiliary layer is, the less conductive it canbe, but if it is very thin (about 1 to 10 μm) it can be substantiallymore conductive than the main lamination adhesive layer withoutcompromising the performance improvements provided by the presentinvention.

For certain applications, it may be advantageous to have a completelysymmetrical structure, with an equally thick layer of laminationadhesive on either side of the electro-optic layer. This structureshould have an effectively identical symmetrical electrical response,which might be expected to reduce or eliminate certain kinds ofelectro-optical artifacts. Such a display structure may be producedusing a symmetrical double release film as described in theaforementioned 2004/0155857.

Alternatively, the electro-optic layer/release sheet subassemblypreviously described may be laminated to a layer of lamination adhesivecoated on a second release sheet, thereby providing a structure (ineffect, a modified double release film) comprising, in order, a firstrelease sheet, an electro-optic layer, a lamination adhesive layer and asecond release sheet. It has been shown to be possible to remove eitherrelease sheet from this modified double release film as a matter ofchoice. The modified double release film is effectively the equivalentof a free-standing electro-optic layer, which can be used to constructdevices in several ways as described in the aforementioned U.S. Pat. No.7,110,164.

This invention may be especially useful in flexible color displays.Although considerable progress has been made in recent years in theproduction of flexible backplanes, including flexible thin filmtransistor (TFT) backplanes, substantial difficulties still remain inthe areas of producing flexible color filter arrays (CFA's), aligningthe pixel electrodes with the CFA elements during assembly of thedisplays, and maintaining this alignment when the display is flexedduring use.

More specifically, one significant area of challenge in producingflexible color displays is in creating the CFA itself. The use offlexible, transparent substrates usually requires low processtemperatures, which can be a problem when a CFA is produced usingconventional photoresists. Lack of dimensional stability duringprocessing makes alignment and registration over large areas difficult,and substrate non-uniformity adds to these problems.

Using a colored lamination adhesive layer as a CFA in accordance withthe present invention provides several advantages. When dyes are used toprovide the colors in the CFA, the dyes can be incorporated into thelamination adhesive polymer, which may be a water-borne polymer latex ora solvent-borne polymer used as a lamination adhesive. A dye ofappropriate solubility must be chosen for the lamination adhesive thatis to be used. Pigments may also be used to color the laminationadhesive. A water- or oil-dispersible pigment should be chosenappropriate for the lamination adhesive to be used. Pigments have theadvantage of not making a major contribution to the dielectric orconductivity properties of the lamination adhesive, and they are notmobile in the lamination adhesive, whereas some dyes are mobile.

It is desirable that the colored lamination adhesive layer be thin(typically 10-50 μm) to reduce the voltage drop across the laminationadhesive layer and hence the driving voltage needed by the display. Insuch a thin lamination adhesive layer, it may be difficult to obtainsufficient extinction using a dye, because of limited solubility of thedye in water or a solvent. Hence, it may in many cases by preferable touse very finely divided pigments (particle size typically 5-50 nm) toallow a thin lamination adhesive layer to have high extinction yetremain light-transmissive.

As already mentioned, during the manufacture of a color display of thepresent invention, the lamination adhesive can be applied directly tothe exposed surface of an electro-optic layer previously coated on asupport, or the lamination adhesive can be applied to a separatesubstrate (which can be a release sheet or an electrode) and theresultant subassembly laminated to the electro-optic layer. This offersa low-cost way of creating an inverted front plane laminate comprisingan integral CFA. Using such an FPL with an integral CFA expands therange of materials available for the front substrate, which can now useessentially any light-transmissive conductive layer or protective layer(formed from glass, plastic or other materials), and does not requirethe ability to be colored to form a CFA. Use of an inverted FPL with anintegral CFA also expands the available range of backplane materials.Furthermore, because the CFA is constructed within the laminationadhesive layer, it is inherently flexible, and having the CFA intimatelylaminated to the electro-optic layer reduces misalignment issues thatmay occur when the display is flexed in use.

A single-color form of the display of the present invention can becreated using an unpatterned colored lamination adhesive and laminationand assembly strategies as set out above.

A variety of methods can be used to apply, coat and/or color thelamination adhesive to form a CFA therein; the method chosen may varywith the size of the individual colored elements of the CFA. Forexample, the lamination adhesive may be deposited by screen printing; acolored polymer latex can be formed having the proper rheology andwetting characteristics to allow it to be printed by silk screen typemethods. Alternatively, the lamination adhesive may be deposited byoffset printing; a colored polymer latex can be formed having the properrheology and wetting characteristics to allow it to be offset printed.Since offset printing is normally effected on webs (as, for example, innewspaper production), offset printing should allow the creation, at lowcost, of sheets of lamination adhesive with an integral CFA ready forlamination to an electro-optic layer. Micro-contact printing may also beused; a colored polymer latex can be formed having the proper rheologyand wetting characteristics to allow the fluid to be micro-contactprinted.

A lamination adhesive layer with integral CFA may also be formed by inkjet or bubble jet printing of a color lamination adhesive. Most ink jetand bubble jet printers apply aqueous colored fluid to a printingsubstrate, using small droplets (typically about 10 μm in diameter) ofthe colored fluid. As discussed in several of the aforementioned E Inkand MIT patents and applications, polyurethane latices are often used aslamination adhesives in electro-optic displays, and the particles insuch a latex at typically of the order of 100 nm in diameter, and hencevery small relative to ink jet droplets. Thus, such latices are entirelycompatible with ink jet and bubble jet printing. Pigment particles ofthe order of 10 nm in diameter can also readily be carried in ink jet orbubble jet droplets if properly suspended, while dyes and solvatedpolymers can easily be carried in such droplets. Finally, it should benoted that ink jet and bubble jet printing are low temperatureprocesses, and thus pose a low risk of dimensional stability problemswhen used to effect patterning on plastic substrates.

A lamination adhesive layer with integral CFA may also be formed by inkjet or bubble jet printing of a dye on to a preformed layer of alamination adhesive. Depending on the type of dye used, it may bepossible for the dye to be printed on and/or diffuse into the laminationadhesive layer. The effect that the ink jet fluid has on the electricalproperties of the lamination adhesive and its lamination characteristicsshould be taken into consideration.

A lamination adhesive layer with integral CFA may also be formed by aresist process. If the lamination adhesive is curable (cross-linkable)in a local area, a difference in solubility to common solvents can becreated to allow patterning. For example, polyurethane-polyacrylatelatices are known which can be cured with ultraviolet or visibleradiation. Such materials can be cured with a laser, or via a photomaskor other process to form a rubbery material that it not easilydissolved. Unexposed areas of the polymer can then be washed away, andthe operation repeated to form the various colored elements of the CFAsequentially. Depending on the characteristics of the laminationadhesive, the flatness of the electro-optic layer, and the degree ofcuring, this patterning could be done on a release sheet to form the CFAas a separate subassembly before lamination of the CFA to theelectro-optic layer. Alternatively, the patterning could effecteddirectly on the electro-optic layer if the process conditions requiredare compatible with the electro-optic layer. If it is possible to createsufficiently high temperatures in a local area without damage to othercomponents of the display, thermal curing could be used in place ofradiation curing.

The patterning need not be limited to a CFA sub-pixel array. Thelamination adhesive could be patterned as if it were a color overlaygraphic (for example, via screen printing) and laminated directly to theoptical layer.

Additives other than coloring materials can usefully be incorporatedinto the lamination adhesive layer used in the display of the presentinvention. For example, ultraviolet absorbing compounds (for example,Tinuvin—Registered Trade Mark) can be incorporated into the laminationadhesive to protect the electro-optic layer from ultraviolet exposure.Such incorporation of an ultraviolet absorber into the laminationadhesive may eliminate the need for an ultraviolet filter layer to beapplied to the front protective layer of a display. Similarly, adiffusing layer can be created by incorporating a light-scattering ordiffracting material (for example, glass beads) into the laminationadhesive, thus creating a display with a matte appearance.

Regardless of the exact method used to form the lamination adhesive withintegral CFA, it is necessary to register the colored elements of theCFA with the pixel electrodes of the backplane. This can be accomplishedby placing alignment marks on the CFA side during the printing processor creating such marks afterwards using an optical registrationmechanism.

The present invention has the advantage that forming a color filterarray in a lamination adhesive layer disposed between the electro-opticlayer and the viewing surface of the display places the color filterarray close to the electro-optic layer, thus minimizing parallaxproblems. The color filter array is also provided in a flexible polymerlayer already present in the display. Alignment of the color filterarray with the backplane electrodes is easily effected and, in the caseof flexible displays, maintained as the display is flexed in use. Thechoice of material for the front protective and similar layers isexpanded, since this layer does not need to be capable of incorporatingor supporting a color filter array. Methods are available for patterningthe color filter array at a variety of resolutions, depending uponperformance and cost requirements. Some of these methods can be carriedout on continuous webs of material and can create an inexpensive frontplane laminate with an integral color filter array. Additionaladditives, such as ultraviolet absorbers, can be incorporated into thelamination adhesive layer, thus simplifying the requirements for otherlayers of the display. For certain applications requiring a singlecolor, the use of a colored lamination adhesive is an inexpensive way tooffer a wide variety of colors.

The question of providing ultraviolet absorbers (filters) in thedisplays of the present invention, and in similar electro-opticdisplays, especially thin flexible electro-optic displays, deservesfurther consideration, since a variety of types of electro-optic mediaare sensitive to ultraviolet radiation. There are three basic approachesto providing the necessary ultraviolet absorbing layer. In the firstapproach, an ultraviolet absorbing dye is incorporated into a polymericlayer which forms the protective layer (front substrate) of the display.In the second approach an ultraviolet absorbing material is coated as aseparate layer on to one (or possibly both) surfaces of the frontsubstrate. Such ultraviolet absorbing coatings are well known in thedisplay industry and hence it is well within the level of skill in theart to provide such coatings on the polymeric films typically used asfront substrates in the present displays. Since the surface of the frontsubstrate facing the electro-optic layer will typically carry an ITO orsimilar electrode, it may be preferable to coat the ultraviolet absorberon the other (normally exposed) surface of the front substrate. In thethird approach, the ultraviolet absorber is included in an adhesivelayer. Incorporation of the absorber in the front adhesive layer hasalready been described. However, in many cases the front substrate usedin the present displays may be a complex multilayer structure whichneeds to be assembled via at least one lamination operation using alamination adhesive and it may be more convenient to include theultraviolet absorber into the lamination adhesive used to assembly sucha multi-layer front substrate.

As will readily be apparent to those skilled in the technology ofelectro-optic displays, the displays of the present invention, whethermonochrome or colored, may incorporate any of the optional features ofprior art electro-optic displays described in the aforementioned U.S.Pat. Nos. 6,982,178 and 7,110,164 and Publication No. 2004/0155857.Thus, for example, the displays and inverted front plane laminates ofthe present invention may incorporate any of the various conductivevias, edge seals, protective layers and other optional featuresdescribed in these published applications.

It has been found that the electro-optic performance of displaysproduced from the prior art FPL's and from the inverted FPL's of thepresent invention are similar, despite the fact that the latter displayshave an additional layer (the lamination adhesive layer) between theelectro-optic layer and the viewing surface of the display. Thefollowing Examples are given, though by way of illustration only toillustrate the various aspects of the present invention and theimprovements in performance which can be achieved in displays of thepresent invention.

Example 1 Production of Experimental Displays

A slurry containing electrophoretic capsules in a polymeric binder wasprepared substantially as described in U.S. Patent Publication No.2002/0180687, Paragraphs [0067] to [0074], except that Dow CorningQ2-521 “Super Wetting Agent” was added as a coating aid. The slurry wasthen bar-coated on to the aluminum-coated surface of an aluminized PETrelease sheet. In one procedure, a 20 μm layer of a lamination adhesivehad previously been coated on to the same aluminized surface; thislamination adhesive was of the type described in U.S. Pat. No.7,012,735, and was doped with 20,000 ppm of tetrabutylammoniumhexafluorophosphate. In a second procedure, the same 20 μm layer of thesame lamination adhesive was laminated to the ITO-covered surface of aPET/ITO release sheet, and the resultant sub-assembly laminated to theelectrophoretic layer/release sheet subassembly, the adhesive layerbeing laminated to the electrophoretic layer.

The lamination adhesive used was known to give relatively good lowtemperature performance, but to give displays with poor room temperatureresolution.

The two structures produced as described above were when used to makeseveral different kinds of displays by three differing procedures, asfollows:

1. The release layer covering the electrophoretic layer was removed, andthe remaining layers (PET/ITO/lamination adhesive/electrophoretic layer)were laminated to a second layer of the same lamination adhesive on afurther release sheet to give a substantially symmetrical structure withthe electrophoretic layer disposed between two similar laminationadhesive layers. The further release sheet was then removed from thesecond lamination layer, and the remaining layers laminated to a 2 inch(51 mm) square carbon black coated backplane to give a functioningexperimental single pixel electrophoretic display, designated “FPL1”.

2. It was found that, with some care, the release layers on thedouble-release structure could be peeled from either side. If therelease sheet covering the electrophoretic layer was removed, theremaining layers could (surprisingly) be laminated directly to ITO on apolymeric film to give a structure (designated “FPLnorm” identical toprior art, not inverted, FPL.

3. Alternatively, after removal of the release sheet covering theelectrophoretic layer, the remaining layers could be laminated to asecond layer of the same lamination adhesive on a further release sheetto give a second substantially symmetrical structure, differing from thesymmetrical structure produced by Procedure 1 only in which side of theelectrophoretic layer was smooth, the smooth surface being of coursethat adjacent the coating support. The release layers on either side ofthe resultant structure could be peeled and the remaining layerslaminated to the ITO-covered surface of an ITO/PET film to give eitherone of two related front plane laminates, designated “FPL1′”, which wasidentical in structure to the aforementioned FPL1, and “FPL1″”, with theelectrophoretic layer inverted relative to FPL1 and FPL1′.

Each of FPLnorm, FPL1, FPL1′ and FPL1″ was laminated to a carbonbackplane in the manner already described to give an experimental singlepixel electrophoretic display. (Note that the surface of the carbonblack backplane was sufficiently rough to preclude preparation of aninverted structure of the present invention with no lamination adhesivelayer between the electrophoretic layer and the backplane.) Similarexperimental displays were also produced using ITO-on-glass backplanesto allow testing of resolution as described in Example 3 below, but inthis case a true inverted structure was provided since the ITO-on-glassbackplane was sufficiently smooth to allow lamination of theelectrophoretic layer with no intermediate lamination adhesive. Alldisplays were incubated at 30 per cent relative humidity for 5 daysbefore the electro-optic testing described in Example 2 below.

The Table below summarizes the structures produced on the carbon blackbackplanes; in this Table, “BP” denotes the backplane, “LA” a laminationadhesive layer and “ELP” the electrophoretic (capsule-containing) layer:

TABLE Code Structure Remarks Control BP/LA/ELP/ITO Slurry coated on ITO,made into FPL FPLnorm BP/LA/ELP/ITO Made from electrophoretic layer onrelease as described above FPL1 BP/LA/(smooth)ELP/LA/ Symmetricalstructure, with ITO; ITO lam 1st smooth side of the electrophoreticlayer toward backplane FPL1′ BP/LA/(smooth)ELP/LA/ Symmetricalstructure, with ITO; Release lam 1st smooth side of the electrophoreticlayer toward backplane; differs in order of lamination from FPL1 FPL1″BP/LA/ELP(smooth)/LA/ Symmetrical structure, viewed ITO; Release lam 1stfrom rough electrophoretic layer side, made by lamination to adhesive onrelease, removing “rough” side release, and laminating to ITO/PET

Example 2 Electro-Optic Tests

The experimental displays prepared in Example 1 above were drivenbetween their extreme black and white optical states using ±15 V, 500millisecond drive pulses and the reflectivities of the two extremeoptical states measured. Testing was effected both at room temperature(20° C.) and at 0° C. FIG. 4 of the accompanying drawings shows thedynamic range (the difference between the extreme black and white statesmeasured in L* units (where L* has the usual CIE definition:L*=116(R/R ₀)^(1/3)−16,where R is the reflectance and R₀ is a standard reflectance value). Ineach case, the left column shows the results obtained at 20° C., whilethe right column shows the results obtained at 0° C.

From FIG. 4 it will be seen that the control and the “inverted”structure of the present invention (the two pairs of columns at theleft-hand side of FIG. 4, the inverted layer being inverted in the sensethat the electrophoretic layer is inverted) show very similar results atboth 20° C. and 0° C. The symmetrical structures also show similarperformance at 20° C. but show poorer performance at the lowertemperature because of the second layer of lamination adhesive.

Kickback or self-erasing (a movement of the optical state away from theextreme optical state following the end of the drive pulse) and dwelltime dependency (the variation of one extreme optical state dependingupon the period for which the pixel has remained in the opposite extremeoptical state before the transition to the one extreme optical state)were similar for all the experimental displays.

Example 3 Resolution

The resolution of the control displays and those of the presentinvention was evaluated by microscopic examination of the displaysformed on glass backplanes; FIG. 5 shows the differences that wereobserved. As indicated in FIG. 5, each of the displays used in thesetests comprised two pixels separated by a 100 μm gap having no ITO layerand hence non-switching. The control display (conventional FPLstructure) is shown on the left-hand side of FIG. 5 and the display ofthe present invention on the right-hand side. Each side of FIG. 5 is acomposite of three separate micrographs of the relevant display. Theupper section of FIG. 5 shows the display with the right pixel switchedto its white extreme optical state, while the lower section of FIG. 5shows the display with the right pixel switched to its black extremeoptical state.

It can be seen from FIG. 5 that, in the control display, blooming causedswitching across the entire width of the inter-pixel gap, i.e., theblooming is at least 100 μm. On the other hand, in the inverted FPLdisplay of the present invention, blooming is less than one capsulewidth (less that 20 μm) and the inter-pixel gap is clearly visible inboth the upper and lower portions of FIG. 5.

These results illustrate the substantial advantage in resolution whichcan be achieved by using the inverted FPL structure of the presentinvention, while maintaining the low temperature electro-optic responseof the display. Thus, the present invention allows both good roomtemperature resolution and good low temperature performance to beobtained from the same display, thus avoiding the compromise betweenthese two performance parameters required in prior art displays.

Numerous changes and modifications can be made in the preferredembodiments of the present invention already described without departingfrom the scope of the invention. For example, although the displays ofthe present invention shown in FIGS. 2 and 3 use red/green/blue colorfilter arrays, the displays of the invention may also usecyan/magenta/yellow or red/green/blue/white color filter arrays.Accordingly, the foregoing description is to be construed in anillustrative and not in a limitative sense.

The invention claimed is:
 1. An electro-optic display comprising, inorder: a backplane comprising a plurality of pixel electrodes; a layerof a solid electro-optic medium; an adhesive layer; and at least one ofa light-transmissive protective layer and a light-transmissiveelectrically-conductive layer.
 2. An electro-optic display according toclaim 1 wherein the electro-optic layer is in direct contact with thebackplane.
 3. An electro-optic display according to claim 1 furthercomprising an auxiliary adhesive layer interposed between the backplaneand the electro-optic layer.
 4. An electro-optic display according toclaim 3 wherein the auxiliary adhesive layer has a thickness not greaterthan about 10 μm.
 5. An electro-optic display according to claim 3wherein the auxiliary adhesive layer has a thickness not greater thanabout one half of the thickness of the adhesive layer on the opposedside of the electro-optic layer.
 6. An electro-optic display accordingto claim 1 wherein the adhesive layer comprises a least two sectionshaving differing colors.
 7. An electro-optic display according to claim6 wherein the adhesive layer has sections having at least threedifferent colors.
 8. An electro-optic display according to claim 7wherein the adhesive layer has yellow, cyan and magenta, or red, greenand blue, or red, green, blue and clear sections.
 9. An electro-opticdisplay according to claim 7 wherein the adhesive layer is colored usingat last one pigment.
 10. An electro-optic display according to claim 9wherein the pigment has an average particle size in the range of about 5to about 50 nm.
 11. An electro-optic display according to claim 1wherein the adhesive layer comprises at least one of an ultra-violetabsorber and a light-scattering or light-diffusing material.
 12. Anelectro-optic display according to claim 1 comprising both alight-transmissive protective layer and a light-transmissiveelectrically-conductive layer, the electrically-conductive layer beingdisposed between the protective layer and the adhesive layer.
 13. Anelectro-optic display according to claim 1 wherein the electro-opticmedium comprises an electrochromic or rotating bichromal member medium.14. An electro-optic display according to claim 1 wherein theelectro-optic medium comprises an electrophoretic medium having aplurality of electrically charged particles disposed in a fluid andcapable of moving through the fluid on application of an electric fieldto the electro-optic medium.
 15. An electro-optic display according toclaim 14 wherein the electrically charged particles and the fluid areencapsulated within a plurality of capsules or cells.
 16. Anelectro-optic display according to claim 14 wherein the electricallycharged particles and the fluid are present as a plurality of discretedroplets held within a polymeric continuous phase.
 17. An electro-opticdisplay according to claim 14 in which the fluid is gaseous.